drm/radeon: fix check order in radeon_bo_move

Reorder the code to fix checking if blitting is available.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 28a68f82 ("drm/radeon/ttm: use multihop")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/403847/